[AstroNet] Fwd: Measuring the Astronomical unit by parallax measurement on Mars

[Christian Hettlage]

Dear All,

Please find below an opportunity for measuring the distance to Mars.  
Please contact John directly in case you are interested. Thanks!

Kind regards,

Christian

> Von: "John Clark" <john.clark at johnclarkastronomy.co.uk>
> Datum: 3. Oktober 2009 14:53:32 GMT+02:00
> An: <assa at saao.ac.za>
> Betreff: Measuring the Astronomical unit by parallax measurement on  
> Mars
> Antwort an: <john.clark at johnclarkastronomy.co.uk>
>
> Dear Sir,
>
> I am a UK-based amateur astronomer with an interest in solar system  
> measurements.  I have spent some time over the last few months  
> estimating the length of the astronomical unit by timing eclipses of  
> Jupiter’s moon Io, essentially reversing an observation by Roemer  
> who discovered the finite speed of light by timing such eclipses.  I  
> measured the speed of light in a university laboratory, and back- 
> calculated the AU from my own observations of Io over a couple of  
> years.
>
> This method worked, but it is difficult to get an accurate value.   
> My value was 20% low compared to the accepted value.
>
> My impression is that the method of Cassini and Richer (1672) is  
> likely to be more accurate, but it requires simultaneously measuring  
> and comparing the position of Mars from two faraway locations on  
> Earth.  This measurement would have to be done to within at least  
> one second of arc.  This is not as hard as it sounds, but it  
> obviously isn’t trivial.
>
> I wondered if there is anyone in South Africa who is interested/ 
> crazy enough to collaborate with me to try this measurement during  
> the forthcoming opposition of Mars?  It would then be at its closest  
> to Earth, so that the parallax would maximise.
>
> Based on a rough estimate, it is about 10,000 km from the UK to SA  
> over the Earth’s surface, so the chord length (straight line  
> distance) is about 9,000 km.  Mars will be at about 100,000,000 km  
> away at its closest approach, so we are looking for a parallax of  
> 9,000 parts in 100,000,000 or about 18.6 arc seconds. A measurement  
> good to 1 arc second would therefore have an error of 1 in 18.6 or  
> 5.4%.  Obviously a measurement good to 0.5 arc sec would have half  
> that error.  With good amateur equipment that should be feasible.
>
> I can help with the maths needed to post-process the photos.
>
> Yours sincerely,
>
> John.
>
> Dr John D. Clark
>
> 323 Wootton Road
> King's Lynn
> Norfolk
> England
> PE30 3AX
>
> +44 1553 679378
>
> john.clark at johnclarkastronomy.co.uk      http://www.johnclarkastronomy.co.uk
>
> Measure Solar System Objects and their Movements for Yourself! by  
> John D. Clark
>
>

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